When a semiconductor component is turned off from its conducting state to its blocking state, as is known firstly a space charge zone forms proceeding from the reverse-biased pn junction, the voltage present at the semiconductor component being taken up in said space charge zone. If the semiconductor component is a diode, by way of example, then the space charge zone propagates proceeding from the pn junction of the diode in the direction of the cathode. In the “back part” of the diode, that is to say in the area before the cathode, part of the originally stored charge is then initially still present, but is reduced with increasing propagation of the space charge zone in the subsequent period. During this subsequent period during which the space charge zone propagates further, a so-called “tail current” flows, which is chopped only when the space charge zone arrives at the highly doped zone upstream of the cathode, that is to say at an nn+ junction. Once the space charge zone abuts on said nn+ junction this leads to chopping of the tail current. This sudden current chopping results in excessive voltage increases on account of the inductance and oscillations present in the electric circuit.
One example of this is shown in FIG. 6 for a 3.3 kV diode. At an instant of approximately t=3E-6 s after the turn-off of the diode, the tail current iT is chopped, which results in oscillations of the current i (A) still flowing overall between anode and cathode. Owing to the high di/dt gradient, this chopping of the tail current also generates voltage spikes, which are extremely undesirable since they may lead to the destruction of the component. It is necessary, therefore, as far as possible to avoid current chopping on account of the space charge zone abutting on nn+ junctions in the event of the turn-off of a semiconductor component.
The above facts and measures undertaken previously with regard to these facts will be explained below with reference to FIGS. 7a and 7b, in which the doping profile for acceptors NA and for donors ND is plotted using solid lines and the profile of the electric field E that results from said doping profile is plotted using dashed lines as a function of the depth w of the semiconductor component. For the electric field E, a distinction is made here between a static field E(stat) and a dynamic case E(dyn) occurring particularly when the polarity is turned off. The width or depth of the weakly doped base zone, which essentially takes up the voltage, is indicated by WB. The highest field strength, achieved in the static case, is Ec.
FIG. 7A reveals the p-doping of the anode zone, the n−-type doping of the voltage taking-up region and the n+-type doping of the highly conducting zone before the cathode. The curve E(dyn) indicates the situation in which the space charge zone abuts on the highly doped n+-type zone when the component is turned off. The chopping of the reverse current as shown in FIG. 6 occurs in the event of this abutting. The curve E(stat) for the static case is situated considerably higher since the phenomena explained with reference to FIG. 6 do not occur here.
The voltages that can be taken up respectively in the static case and dynamic case correspond to the areas beneath the curves E(stat) and E(dyn), that is to say the field strength E(w) integrated over the width w.
To summarize, it emerges, then, that a component having the doping profile shown in FIG. 7A has a relatively high static blocking capability and a relatively low dynamic blocking capability, the dynamic blocking capability being the voltage at which reverse current chopping occurs.
One possibility for increasing the dynamic blocking capability might consist in making the width of the voltage taking-up region, that is to say the width wB of the n−-doped region, so large that the electric field does not reach the nn+ junction, that is to say the width wB, at the highest voltage with respect to which the component is commutated. In the case of components having a high blocking capability for an area of use of greater than 2 kV, however, the doping must be chosen to be low enough to achieve a sufficient stability against cosmic radiation. Such a low doping permits the gradient of the electric field to become small, however, and leads to a large extent of the space charge zone. Moreover, a large width of the voltage taking-up region, also called central zone, leads to high on-state and/or switching losses. In this case, said on-state and/or switching losses increase approximately proportionally to the magnitude of wB. Therefore, it is not possible to choose the most suitable values from standpoints of the blocking capability for the width wB.
As is known, there are IGBTs or thyristors which have a more highly n-doped layer as a so-called field stopping area or “buffer” before a p-conducting collector zone or anodal emitter zone. Such a field stopping area can be produced by proton irradiation from the collector or anode side and subsequent annealing (in this respect, cf. “13 kV Rectifiers: Studies on Diodes and Asymetric Thyristors”, Proceedings ISPSD 2003, pp. 122-125).
The situation manifested in a component comprising such a field stopping zone is illustrated for the case of a diode in FIG. 7B. As can be gathered from said FIG. 7B, the field stopping zone leads to a higher gradient of the electric field strength E on account of its increased doping. The area beneath the curve E(dyn) thus becomes considerably larger than in the case of a component without a field stopping zone. That is to say that the space charge zone abuts on the nn+ junction only at a higher voltage or—for the case where the field stopping zone is sufficiently highly doped—does not abut at all on the nn+ junction. In the dynamic case, therefore, the semiconductor component configured in this way can block higher voltages.
One disadvantage of a field stopping zone, the depth of which is indicated by WBuf in FIG. 7B, is that the area beneath the curve E(stat) is significantly smaller than in the case of a semiconductor component without a field stopping zone according to FIG. 7A because the zone behind the field stopping zone or between the latter and the highly doped n+-type zone can contribute scarcely or not at all to taking up the electric field.
A semiconductor component in which a curve E(dyn) according to FIG. 7B is applicable in the dynamic case, while the profile of the curve of E(stat) from FIG. 7A is maintained in the static case, would be inherently desirable. This condition is apparently not met by existing semiconductor components.
When a silicon body is irradiated with high-energy electrons or with H+ or He++ atomic nuclei (ions), in the silicon a series of different centers Z arise in the area between the valance band V and the conduction band L of silicon, as is illustrated schematically in FIG. 8. Some of these centers are used as recombination centers. Irradiation with H+ ions, that is to say proton irradiation, also gives rise to a center which acts as a fixed donor and can be used for example for producing a field stopping zone, such as for the field stopping zone “Buffer” in FIG. 7B.
Apart from the fixed donor assigned to the proton irradiation, the irradiation with e.g. protons, He++ nuclei carbon atoms or electrons and subsequent annealing at a temperature of more than 220° C. give rise specifically to the centers Z shown in FIG. 8, namely double vacancies VV or E (230 K) at −0.24 eV, −0.43 eV and +0.19 eV and a center made up of an association of an oxygen atom with a vacancy, namely a center OV or E (90 K) at an energy level of −0.17 eV, these centers acting as recombination centers.
The so-called K center (COVV), which is described as an association of a carbon atom, an oxygen atom and two vacancies, also arises in all types of irradiation with a subsequent annealing process, that is to say in the event of irradiation with high-energy electrons, with H+ nuclei, He++ nuclei or carbon ions. This center is only very weakly effective as a recombination center. It lies at an energy level of +0.355 eV and is also designated by H (195 K).
The K center COVV has been known hitherto primarily owing to its disturbing properties. Thus, DE 187 09 652 A1, for example, reports that a homogeneous distribution arises at K centers after an electron irradiation. These K centers act as a temporary donor: they are positively charged directly after current flow. This charge state lasts a few 100 ns to a few μs depending on the temperature. At this time in which the K centers are effective, the effective basic doping of the semiconductor body doped with them is temporarily raised. On account of such an increased basic doping, a semiconductor component undergoes transition far below its static reverse voltage to an avalanche breakdown. Such a breakdown triggers a high-frequency oscillation, the dynamic IMPATT oscillation. Said IMPATT oscillation disappears after a few microseconds, the semiconductor component generally not being destroyed, however.
It is necessary to avoid an IMPATT oscillation on account of the strong electromagnetic interference emission. For this reason, the permissible number of K centers is limited in the case of the power diode described in DE 197 09 652 A1. This means that electron irradiation can be employed without causing disturbing oscillations.
The publication “Analysis of Dynamic Impatt Oscillations caused by Radiation Induced Deep Centers” Proceedings ISPSD 2003, explains how it is possible to demonstrate the doping effect of K centers by the production of a local profile of defects in the implantation of He++ nuclei. The disturbing IMPATT oscillations are likewise used in the process. FIG. 9 shows a profile of the various centers after a helium implantation and annealing in the region of 350° C. It can clearly be seen from FIG. 9 that the COVV centers exceed the OV and VV centers in the concentration K (cm−3).
In the above publication, the He++ nuclei are implanted from the anode side. Therefore, the distance from the anode is plotted on the abscissa of FIG. 9. However, this publication, too, refers to the disturbing influence of the K centers, which should be avoided as far as possible.
Consequently, the prior art gives the person skilled in the art the clear instruction that the occurrence of K centers should be prevented as far as possible in semiconductor components owing to their disturbing influence.